[Nebster]

Greetings all,

Now that things are mostly assembled, mostly working, and we are a few days away from becoming full time RVers again, I wanted to start a thread to document some of the electrical rejigging I have been undertaking in coach 1234, a.k.a. "the Enterprise."

I basically decided I wanted a new hobby for 2017 and 2018, so I started designing and procuring this system last summer.  It uses 96 LiFEPO4 (hereinafter "LFP") cells connected in six paralleled, sixteen-cell strings.  Each string has about 5kWh of energy storage, making a total pack of about 31kWh, or as I like to say to folks, "about half a Tesla."

Here are some of the cells on my test bench, getting balanced, prior to installation:

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And here is a "string" of 16 cells getting prepped to go in the battery tray.



I had to remove and modify the tray to get about an extra inch of clearance on the lower trays, versus the Newell stock 8DLs we know and love.

[Nebster]

Here are all six strings, fully wired up.  Each string goes through a dedicated contactor that can isolate the string, either automatically if there is a fault detected on the string itself, or manually if I need to take one string out for maintenance.



Each "cube" of 16 cells is sitting on a neoprene vibration isolation pad and is strapped together at the top with solid bus bars.  You can see gaps between the cells -- this is a newer style of cell made with aluminum casings, and those casings are designed to be electrically isolated via the light blue spacers at the top and bottom.  The gaps also allow air to circulate, which helps to keep the total pack temperature more even.

It remains to be seen whether this mounting approach will be sufficient mechanically.  It is one of my least favorite parts of the design, and I do have the option to remove the Newell trays entirely and place all 96 cells on the floor of the bay directly.  If I find that there is any shifting going on, I may have to switch designs down the road.  Stay tuned on that!

Each string is bussed up to bars on the top there, and then those bars send ~52VDC nominal over to the inverter bay re-using the old, dual 4/0 cables that were already in place on our Newell.

There is a monster contactor between the busbar and the inverter bay that can interrupt the entire pack in the event of a thermal or electrical fault detected in the battery. Just like the string-level contactors, I can also manually disconnect the pack for maintenance.

[Richard]

So, you are using a different inverter?

And, what are you doing for 12VDC power for lights and what not?

What other systems are you using for battery protection and battery monitoring?

Love your project !

[folivier]

Nice! So with a 52 volt system how are you getting down to 12 vdc? And how will you charge back up to 52 vdc?
And is that 31Kwh at 52 volts?
I did a simpler system with 4 1000amphour cells which are still going strong in one of our gurus coaches.

[Richard]

Yeah Ben,

I have the system that Forrest built.

So, I am going to ask you a zillion questions. Hope you don't mind.

[Nebster]

The inverter system, shown here with most of the new equipment installed, has been refactored to handle the new pack.



First of all, the entire 240V, 120V, and also the 12VDC house loads are all run off of this inverter now. That means I had to pull a new supply wire from the rear load panel to the inverter bay, and the shore cord and generator supply wires also terminate in the inverter bay now. There is no standalone transfer switch in this system; the Victron inverter you see on the forward wall there can switch all of the AC inputs internally.

The inverter/charger you see is a 230V/240V inverter that can produce 15kVA of AC continuously at 75F and peak at 30kVA for up to 30 seconds. Because we need 120V, its output goes to the autotransformer you see on the rear wall. That AT can sink 32A of neutral return current, which means up to about 4kW of "imbalance can exist between the two 120V legs of the house power side.

There are 2x1500W 120VAC-to-12VDC converters wired onto the house side of this system. These converters provide the 12VDC for the house loads (Spyder panels, miscellaneous pumps, etc.). Some of the "automotive" side loads are still powered off the pair off the chassis 8DLs in the rear of the coach. Also, the generator starter motor is also now tied to those rear batteries.



So far, the largest load I have been able to create is about 16000W: the aquahot, all of the electric space heaters, several of the roof heat pumps, and some lighting. Smooth sailing.

[Richard]

Amazing!

[Nebster]

The inverter-charger shown above can charge at 200A, or theoretically about 10kW. In practice, on a residential 50A circuit there is not enough headroom at NEC-derated 80% continuous to reach that level. I have not checked to see if the genset is safely wired for >50A, although I imagine it is since the overcurrent breaker is sized at 80A.

Anyway, charging LFP at higher rates is not as good for the chemistry, so we will settle for about 150A @ 52V, or ~0.25C. We will try never to let the pack get below 20% SOC, so a full topoff should take around three hours of generator runtime.

Historically we have spent a lot of time camping in places where we have access to a 120V shore source. Campsites with 30A, driveways of friends with 20A or even 15A, etc. So, I have added a secondary, smaller 120V Victron IC for charging off these smaller sources.

The smaller Victron is mounted one bay forward, at the interior of the curbside bay, where we had an extra twelve inches back there that our slideout tray doesn't use. (Our house vacuum and solar charge controller are back there, too. More on that in a bit.)

The small inverter can charge at 35A, or ~1800W, or ~43kWh per day. As long as our average demand per day stays below that, then we can run indefinitely off a 20A circuit without needing to resort to a generator.



Here's a view of both sides from the mobile app. Right now we are plugged into 240V at the house, I have the charger software-limited to 15A@240V for some testing, and the 120V inverter is idling.

In an emergency, were we to lose the 240V inverter for some reason, we could drive up to 3kW of loads off the small inverter. So there is a small redundancy benefit that comes along with the hassle of a second, smaller system.

[Nebster]

So, you are using a different inverter?

Yes, you got to my thread too fast for me. 

And, what are you doing for 12VDC power for lights and what not?

Newells with incandescent lighting have absolutely enormous DC draws.  Upwards of 1200W on my coach!  I am slowly replacing with LED, but I don't like the performance of most 12V LEDs on the Spyder (voltage-based) dimming system.  I started with one 1500W DC power supply, but I decided to add a second one on standby so we can handle peaks better, degrade gracefully when the inverter bay gets super hot in the summertime, and so that we don't lose most of the coach if one of the 12V supplies bites the bullet.

What other systems are you using for battery protection and battery monitoring?

Primarily just bottom-balance and conservatism.  I am using a custom Zeva cell monitoring solution that can trigger start and stop charge as well as pack disconnect on cell-level low and high voltage.  I actually haven't even finished wiring it up on all the strings, but the system is intrinsically pretty safe as it is, even without the extra protection.

Love your project !

Thanks; me, too -- most of the time. 

Nice!  So with a 52 volt  system how are you getting down to 12 vdc?  And how will you charge back up to 52 vdc?
And is that 31Kwh at 52 volts?

Good catch, we use the aforementioned 12VDC supplies to re-generate 12VDC.  That makes for an inefficiency (inverter is ~93% under decent load), but (a) it's kind of a rounding error in this system and (b) a lot of the design of the system is greatly simplified by having a single, unified, controllable load.  Other than the emergency contactors and the control computer, nothing else runs directly off the DC pack.

I did a simpler system with 4 1000amphour cells which are still going strong in one of our gurus coaches.

Cool, are those Winston cells?  I didn't even discover their huge cells until well after I'd placed my order for the ones I'm using.

Yeah Ben,

I have the system that Forrest built.

So, I am going to ask you a zillion questions. Hope you don't mind.

I've heard about that system, I think.    Send me all your questions!

[Nebster]

We've elected to install 4s4p 100W solar panels on the roof.  I know we could fit 20 panels up there, and we might even be able to squeeze 24, but 16 strikes a nice balance between giving us some meaningful energy under most any conditions (we have a coach "baseline" without HVAC of about 250W continuous, unoccupied) and keeping enough of the roof open that I can walk on it.



Here you can see the first few panels going in at the front of the coach.  The mounts are made from 80/20 extruded aluminum rails, some associated "footpads," and VHB tape to secure them to the roof.  They are pretty stable.  (VHB might be my favorite product, ever?) They are designed to raise the panels up flush with the fairing, so that the panels can be brought almost to the edges without any shading. As a side bonus, I figure I could put some storage for brooms, mops, and wrapping paper up there underneath them, if I get desperate for space!

The 4 strings bus up into one combiner box and send about 20A @ 80VDC down through a set of 8AWG cables that Newell ran for me.  Those also land in the inverter bay and onto a small Victron MPPT charger.